The London train bombing has prompted BART Police to increase patrols on the Bay Area Rapid Transit System.
BART Police say there have been no recent credible threats, but they are taking extra precautions.
"We have extra officers working the stations and our trains," said BART Police Deputy Chief Ed Alvarez.
Some riders had concerns, thinking about the September 15th bomb attack on the London underground subway system earlier in the day.
"With so many attacks going on right now it's definitely top of mind and as a mass transit user. t's definitely we want to know what's happening and what the plans are in place to protect us," said Chris Flores, a San Francisco BART rider.
Like the London subway system, where crowds come and go with all sorts of bags, backpacks and suitcases, police say another important piece of the security is for riders also to be more vigilant.
"Things that are out of the ordinary that shouldn't be there, you know that shouldn't be there, report it. That's something that we rely on. Go with your gut feeling. If you see something that looks out of place, call in," said Deputy Chief Alvarez.
People who ride BART regularly say they do have concerns about the Bay Area becoming a target.
"You got poeple coming from around the world. San Francisco is a big attraction so people come from everywhere and you don't know what they got in their minds, what they want to do here. It's a big population," said Earl Trotter, a Vallejo BART rider.
Mass transit is an open system with its many access points and mission to move people around quickly. That poses a huge security challenge according to Joshua Filler, who formerly worked at the Dept. of Homeland Security.
"There are things you don't see, certain detection capabilities that are in place; all designed to create layers of security. But there is still an inherent vulnerability to mass transit that is inescapable if it's gonna function as it's set up to do," said Filler.
Some BART riders say police need to have a more visible presence..
"I do remember when there were the Warriors games, there was Homeland Security at the Coliseum. I was actually very comfortable with that at that time," said Marcel Sheri Joseph, a Union City BART rider.
Some riders say police should not just be in the station, but also visible on the platforms and inside the train cars where a terror attack might be more likely to occur.
"I don't really see many in general and I ride every day Monday through Friday. probably see them maybe once a week walking around," said Sheri Calvert, a Pacifica BART rider.
BART police say the increased presence will not end Friday. They plan to continue the additional patrols indefinitely.